The present invention relates to a method for recording local boundary-layer flow-line directions in liquid media by means of an applied photographic layer.
The dissipation of heat occurring in all electric machines and apparatus gives rise to problems regarding cooling. The methods customarily employed in process engineering for calculating the local heat transfer coefficients of technical importance for the cooling process are, however, usually insufficient. Thus, it is necessary to rely upon experimental determination. This local convection transport coefficient is determined by the state and form of the Prandtl flow boundary layer, defined as a layer-like zone wherein friction between the core flow and the heat-exchanging surface is such that in the layer the flow velocity drops from 99.9 percent of the core flow to zero at the point of direct adhesion to the wall.
Heretofore it has been possible to determine the boundary layer properties such as local thickness, local wall shear stress, local mass and heat transfer coefficient only by individual point-by-point measurement in a time-consuming procedure and with little local resolution.
It is known from U.S. Pat. Nos. 3,774,225; 3,890,835; 3,787,874 and commonly-assigned U.S. application Ser. No. 168,245, filed July 10, 1980, to cover an entire range of local transport coefficients with high location resolution in a single measurement, whereby an efficient flow boundary layer analysis becomes possible.
The wet-film diffusion method known from U.S. Pat. No. 3,774,225 is particularly well suited for a two-dimensional display of the local heat transfer coefficients by reason of the known analogy between mass and heat transfer coefficients. Except for special cases, however, it has not yet been possible to derive the direction and magnitude of the local boundary-layer flow in liquid media from a wet film photogram of the wet-film diffusion method without substantially interfering with the core flow. FIG. 1 shows a boundary layer display, made visible by means of the wet-film diffusion method, of the heat transfer surface of a cooling cell with steady-state flow. The input and output of the coolant are indicated by arrows. Unambiguous information regarding the distribution of the local flow direction of the coolant over the surface cannot be gathered from the recording.
For optimizing problems in special heat transfer cases, in which knowledge as to the direction and amount of the often complicated flow at the heat-exchanging surface is absolutely necessary, the required data must be determined by painstaking probe measurements or by elaborate calculations. For example, it was found necessary to inject dye solutions through fine holes at the walls adjacent to the flow of interest in order to display the local flow direction of boundary flow layers.